Question

I try to write on database in multi-Thread at the same time
but the error occurred in myCommand.Connection.Open();
The error : Object reference not set to an instance of an object.
How can I Solve this problem ?

This example show the problem

private void button1_Click(object sender, EventArgs e)
    {
        new Thread(() =>
        {
            SqlCommand myCommand = new SqlCommand("insert into table(a,b)values(1,'aaa')", Connection);
            myCommand.Connection.Open();
            myCommand.ExecuteNonQuery();
            myCommand.Connection.Close();
        }).Start();
        new Thread(() =>
        {
            SqlCommand myCommand = new SqlCommand("insert into table(a,b)values(2,'aaa')", Connection);
            myCommand.Connection.Open();
            myCommand.ExecuteNonQuery();
            myCommand.Connection.Close();
        }).Start();
        new Thread(() =>
        {
            SqlCommand myCommand = new SqlCommand("insert into table(a,b)values(3,'aaa')", Connection);
            myCommand.Connection.Open();
            myCommand.ExecuteNonQuery();
            myCommand.Connection.Close();
        }).Start();
        new Thread(() =>
        {
            SqlCommand myCommand = new SqlCommand("insert into table(a,b)values(4,'aaa')", Connection);
            myCommand.Connection.Open();
            myCommand.ExecuteNonQuery();
            myCommand.Connection.Close();
        }).Start();
    }

Answer 1

You need a valid connection:

SqlConnection connection = new SqlConnection(...);
connection.Open();

SqlCommand command = new SqlCommand(...);
command.Connection = connection;
command.ExecuteNonQuery();